Increased Item Quantity and Increased Item Rarity are the two stats in Path of Exile that aid players in terms of gathering loot. Players refer to this as Magic Find or MF technically, however, the function of Quantity and Rarity is in different ways. Here’s the difference:
When you farm run or go through grinding, the Increased Item Quantity (IIQ) raises the total number of items dropped by monsters. For instance, if you have a + 100% IIQ, what should’ve been a drop of 2 Blessed Orbs would become 4 Blessed Orbs in effect. As you can see, this raises your chances of acquiring poe items and building your financial capabilities quickly, although we expect to see a whole lot of junk when this stat is in play. If you plan to farm for items and equipment with IIQ in effect, it would be recommended to have a spacious inventory.
Increased Item Rarity (IIR) on the other hand doubles the number of Normal (White), Rare (Yellow) or Brown (Unique) items that you would find as long as it’s in effect. This does not necessarily double the items that you loot but rather gives you a better chance of avoiding junk equipment. This is used in areas where a certain piece of good equipment drops from a boss or whatever. Such as poe exalted orb.
Since the number of drops triggered by the skill and also affects your chances of acquiring good equipment, some people say that IIQ is better than IIR especially if your Exile is adept in killing mobs efficiently. For solo PVE runs to have + 150% IIQ and + 50% IIR, it is common practice. Having these 2 stats at equal levels are rarely beneficial and more often than not, the effects are not evident. It’s a good rule of thumb to either favor IIR or IIQ then support the other.
Of course, it is every Exile’s dream to get as much IIQ and IIR, but given the price tagged for pieces of equipment with these boosts, only seasoned and veterans can afford them. With that being said, if you farm along with a party, since a full roster has generally about +250% of IIQ factored in, then you may consider boosting IIR. But for solo runs, get more IIQ so that your PoE Currency Items builds at a much faster rate along with your chances of obtaining good equipment.